1. Field of the Invention
This invention relates to the direct fabrication of metal parts, and more specifically to a continuous thermal process in which a carbon precursor is used to bind the "green form" part and catalyze a eutectic reducing element in a supersolidus liquid phase sintering (SLPS) process that provides a final part of parent material quality.
2. Description of the Related Art
Historically, three-dimensional parts are machined by removing material from a block to form a final part directly or to form a mold for casting the final part. Direct machining of final parts is rarely cost efficient when done on a production scale. More typically, large scale production uses a casting process that is fairly time and cost efficient and produces cast quality final parts. However, the cost of retooling and machining a new part can be very high, both in dollar and man hour investment and in the delay in getting a new design into production. This can be a significant deterrent to updating and improving the design of the part.
In recent years several new manufacturing methods have been developed under the name Rapid Prototyping and Manufacturing (RPM) and have two things in common. First, the part is modeled as a 3D solid, which is "sliced" into very thin 2D layers and stored in a three-dimensional Computer Aided Design (CAD) file. Data is then transferred from the CAD file to the manufacturing equipment that builds the part layer-by-layer into the three dimensional structure. The concept of `Art-to-Part`, proceeding directly from a CAD file to a prototype part or mold, is extremely attractive to engineers and management alike. Second, the parts are manufactured by adding material where it is needed instead of removing material where it is not needed. This eliminates the need for retooling and machining, and permits complex structures to be made.
True RPM produces a "final" part in a single continuous process. The final part is ultimately a prototype shape or mold. None of the known RPM technologies produce metal parts that can be used in a structure or machine, the metal parts are too fragile and cannot withstand structural loads. If development requires metal parts, the polymer molds are typically used in a multi-step process to cast the metal parts.
At present there are a number of different RPM technologies that fall into one of three categories. Stereolithography (SLA) involves the chemical change of a material using light energy. Selective laser sintering (SLS) sinters a powder using a laser. Deposition techniques such as Solid Ground Curing (SGC), Laminated Object Manufaturing (LOM), Fused Deposition Modeling (FDM), and Ballistic Particle Manufacturing (BPM) use selective deposition of either particles or thin laminates.
In stereolithography a photosensitive liquid polymer is solidified layer by layer. The liquid polymer solidifies when it is exposed to a low power, ultra violet (UV) beam, for example, a UV laser. After each layer is solidified the part is lowered incrementally in the liquid and the next layer can be solidified. This method is restricted to the use of certain solidified polymers that allow solidification by a laser beam and, unfortunately, these polymers are often brittle. Stereolithography was the first commercial RPM-method (1987) on the market and is now the most frequently used.
William T. Carter, Jr. and Marshall G. Jones, "Direct Laser Sintering of Metals, Proc. Solid Freeform Fabrication Conference, Austin, Tex. Aug. 9-11, 1993 describes the basic selective laser sintering process for sintering parts from a binderless metal powder. The SLS process for plastic, metal, ceramic or polymer powders is described in detail in a series of U.S. Pat. Nos. 4,863,538, 4,938,816, 4,944,817, 5017,753 and 5,053,090 that are assigned to the Board of Regents of The University of Texas System. A roller spreads out the powder on a platform and the material is preheated to a temperature just below the melting temperature. An infra red laser beam raises the temperature where consolidation is desired and the powder fuses together. After each layer is solidified, the roller spreads out a new thin layer of powder over the part and the laser scans the next layer of the part in accordance with the 3D CAD file. When the solidification process is completed the green part is surrounded by powder that can be removed with a brush or by compressed air. This powder forms an important support to thin sections during the process. The green part is fairly fragile and thus only useable during the prototyping stages.
A number of modifications to the basic SLS process have been investigated to improve the part's density and strength so that it can withstand some degree of structural loading during prototype testing. None of the known methods provide final parts with parent material quality. Materials that are going to be used in commercial or military products must be qualified, which is a long and costly process. Thus, manufacturers have a strong incentive to use parent metal alloys that have already been qualified.
The RPM process can be augmented by back-infiltrating a secondary liquid metal such as copper via capillary pressure into a partially densified part to provide strength. The infiltration process interrupts the continuity of the RPM process, provides typical densities of only approximately 80% of the theoretical limit, and contaminates the metal alloy to such an extent that it can no longer be considered parent quality material. As a result, the part's mechanical properties such as strength and ductility differ significantly from the parent alloy. The elemental composition is different enough that the resulting alloy would require requalification.
Konig et al. "Rapid Metal Prototyping--New Approaches for Direct Manufacturing of Metallic Parts," Proc. 27.sup.th ISATA, Aachen, Germany Oct 31-Nov. 4, 1994 pp. 281-288 describes an SLS process in which low and high melting point alloy powders are used without the addition of binders. The laser melts the low melting point powder causing it to wet the surface of the high melting point powder and bind the individual particles together. Konig used steel and nickel-bronze alloys. Haynes 230 metal alloy powders undoped and doped with 3% boron by weight to reduce its melting point temperature have also been used. The hot isostatic pressing (HIPing) process is also used to close the small amounts of isolated porosity. The high temperatures required to melt the doped alloy create severe temperature gradients that stress and distort the part during formation.
"Little Wonder Uses SLS.TM. Process & Nylon to Create Functional Hedge-Trimmer Prototype," Spring/Summer 1994 Volume 4, No. 1 describes DTM Corporation's variation on the SLS process in which a polymer powder is blended with a metal powder. The laser melts the polymer powder layer-by-layer in accordance with the 3D CAD file to form a solid but porous green form part. After the green form part is heated to burn out the polymer binder, it is subjected to partial sintering to impart residual strength to the remaining metal powder for subsequent densification via HIPing. This approach produces a very low metal density after burnout of the polymer binder, which results in a lack of control of part dimensions and shape during HIPing and very low strength in components utilizing liquid metal infiltration for final densification.
U.S. patent application Ser. No. 08/530,770 entitled "Free Form Fabrication of Metallic Components" filed Sep. 19, 1995, now U.S. Pat. No. 5,745,834, and assigned to Rockwell International, the Assignee of the present invention, uses a mixture of a parent metal alloy X such as Haynes 230, a metal alloy Y that is identical to alloy X except that it is doped with another alloying element such as boron to lower the melting point, and a polymer binder in the SLS process. The laser melts the polymer binder in accordance with the 3D CAD file to build up a green form part layer-by-layer.
Once the green form part is completed, the binder is eliminated in a vacuum furnace. It is necessary to support the green form part during elimination and the subsequent densification process because the removal of the polymer binder and the partial liquidation of the metal temporarily reduce the integral strength of the part. Support may be provided by a ceramic powder or a structure.
Densification is achieved via liquid phase sintering (LPS) by raising the furnace temperature to melt the doped alloy so that it encapsulates the undoped alloy powder. Finally, a HIPing treatment may be necessary to close any residual porosity and complete the homogenization of the part. This process still requires additional support for the green part and a HIPing treatment.
Known deposition techniques include Solid Ground Curing (SGC), Laminated Object Manufacturing (LOM), Fused Deposition Modeling (FDM), and Ballistic Particle Manufacturing (BPM). SGC is similar to sterolithography in that parts are built up with a photo reactive liquid polymer, but instead of scanning the surface with a laser beam this technique uses an electro static process to mask areas that are not supposed to be solidified. After each layer is solidified the remaining liquid polymer is removed and replaced by a layer of liquid wax that fills pores and cavities. A chilling plate hardens the wax and the surface is milled to the correct thickness. This process is repeated for the next layer. The advantage in this technique is that the wax constitutes a solid ground for each layer which means that there are practically no limitation on geometry.
In the LOM approach, a laser beam cuts the outline of the first layer of the part in a sheet of material and cross-hatches the excess material for later removal. A new layer of the sheet material is placed on top of the first and a laminating roller binds the layer together. The cutting motion of the laser is repeated for this layer. Once all layers have been laminated and cut, excess material is removed to expose the finished model.
FDM uses a spool of modeling filament resembling wire. The system feeds the filament through a heated head and nozzle. Just before deposition, the head heats the thermoplastic filament to a temperature slightly above its solidification state. As it deposits material, the material solidifies.
BPM has developed a technique for 3-D printing in an office environment. The process utilizes a piezoelectric jet head that deposits molten thermoplastic material wherever it is needed. The droplets will first flatten then solidify upon contact with the surface. This process is primarily used for modeling.
In a relatively new development, Randall M. Germann, Powder Metallurgy Science, 2.sup.nd edition, Metal Powder Industries Federation, 1994, pp. 270-283 describes a recently discovered method of densifying an already homogenized but porous metal alloy known as Supersolidus Liquid Phase Sintering (SLPS). As the homogenized alloy is raised above its solidus temperature, a liquid film forms along the grain boundaries and particle necks between grains. This promotes grain boundary sliding of the polycrystalline particles. The disintegration of particle boundaries allows rapid densification due to surface tension and capillary pressure. As the liquid reprecipitates the grains themselves deform to assist in pore removal and final densification. SLPS is superior to conventional LPS in that a single alloy is formed rather than one alloy being encapsulated by another. LPS will homogenize if left long enough at a high enough temperature.